Laser jet printers produce images by retrieving image data, processing the data, and ultimately transferring the image onto a print medium (e.g., paper). Although specific technologies may vary, laser printers in general include the same basic components. For example, laser printers generally include at least one drum assembly, a charging element, a laser scanning unit, a toner chamber, a toner developer roller, a fuser, and at least one cartridge assembly. Essentially, the drum assembly is given an overall negative charge by the charging element. As the drum revolves, the laser assembly emits a laser beam across the surface of the drum to discharge the negative charge on the drum at predefined points. In this way, the laser “draws” the image to be printed as a pattern of electrical charges onto the drum. Once the image is impressed onto the drum, the drum is coated with negatively charged toner, which is generally a fine powder. Color laser printers generally employ four or more “colors” of toner, each with a corresponding drum, to produce images that can comprise a substantially full color gamut. The colors of toner generally used are Cyan, Magenta, Yellow and Black (CMYK). Since the toner has a negative charge, it clings to the discharged points on the drum, but does not cling to the remainder of the negatively charged drum (i.e., the background of the image). With the toner clinging to the pattern impressed portions of the drum, the drum rolls over a positively charged sheet of paper. Because the charge on the paper is stronger than the charge of the image on the drum, the paper pulls the toner powder away from the drum. At this point, the paper is passed through a fuser (e.g., a pair of heated rollers), which melts the toner powder, fusing it with the fibers of the paper.
In general, the quality of an image produced by a laser printer is related to the path or trajectory that the laser beam takes while traveling across the drum. In other words, an image, when processed by the printer, is broken down into a plurality of horizontal line segments, each of which is processed and transferred onto the drum by the laser. Ideally, the path of the laser beam as it impinges on the drum should be a straight, horizontal line running in the “scan direction,” which is generally parallel to the center axis of the drum (i.e., the “ideal” beam path). However, due to an inherent initial offset in the physical configuration of the printer components (i.e., the relationship and alignment of components internal to the laser scanning unit and the alignment of the laser scanning unit to the drum assembly), the path of the laser beam along the drum assembly, rather than being straight and parallel to the drum axis, exhibits a certain amount of bow and tilt. Bow generally refers to the degree of arc in the beam path relative to the center axis of the drum, while tilt refers to the “vertical” offset between one end of the beam path and the opposite end of the beam path. Vertical offset refers to offset in the “process direction,” which is perpendicular to the center axis of the drum.
Another consideration for color laser printers is the concept of “color plane registration,” which is the alignment of each color plane to form a full color image. Each toner color that is used to produce an image is referred to as a “color plane”. Because all of the color planes collectively form the overall image, the quality of the image is dependent upon the alignment of each of the color planes relative to one another. An image having misaligned color planes can appear to lack sufficient sharpness and clarity and can appear disjointed. Proper color plane alignment is often complicated when the laser beam path exhibits bow and/or tilt.
One known method for reducing the effects of bow and tilt is to calibrate and align the laser and the drum assemblies at the manufacturing facility according to tight specifications. This approach, however, can be costly and labor intensive, which ultimately adds to the cost of the printer. Accordingly, the embodiments described hereinafter were developed in light of these and other drawbacks associated with laser printer designs.